Development of a Deep Indo‐Atlantic Supergyre Prior to the Eocene/Oligocene Transition

Today, ocean circulation is characterized by southward‐flowing North Atlantic Deep Water and northward‐flowing Antarctic Bottom Water, with a mixture of water masses filling the Pacific and Indian Oceans. Also, a Southern Hemisphere supergyre (i.e., an inter‐basin wind‐driven current) facilitates exchange of water among South Pacific, Indian, and South Atlantic Oceans subtropical gyres. The ocean circulation in the Eocene was different. Here we present new benthic foraminiferal oxygen and carbon stable isotopes (δ 18 O bf and δ 13 C bf ) from Ocean Drilling Program (ODP) Site 1090 (Agulhas Ridge) spanning the middle and early late Eocene that extend a published late Eocene‐early Oligocene record (Pusz et al., 2011, https://doi.org/10.1029/2010pa001950 ) into a time characterized by the evolution of the Drake Passage and Tasman Gateway. The comparison of the Site 1090 combined data set with δ 18 O bf and δ 13 C bf from the Southern, Atlantic, and sub‐Antarctic Indian Oceans confirms the presence of a water mass with higher δ 18 O bf at depths <2,000 m and a water mass with lower δ 18 O bf at depths >2,000 m, as already reported. We interpret these signals to be evidence of a deep supergyre at depths <2,000 m transporting salty higher δ 18 O bf waters originating from the Indian Ocean into the Southern and eastern South Atlantic Oceans. Cold waters that formed around Antarctica spread northward, bathing sites >2,000 m. As the Drake Passage and Tasman Gateways opened and deepened and the Antarctic Circumpolar Current developed, deep‐water formation became more prominent at higher latitudes, while the supergyre became restricted to shallower depths.